win: map UV_FS_O_EXLOCK to a share mode of 0

This is necessary to enable writing past the MBR of a raw block device.

Fixes: https://github.com/libuv/libuv/issues/1605
PR-URL: https://github.com/libuv/libuv/pull/1613
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>

docs/src/fs.rst

@@ -409,7 +409,9 @@ File open constants
     Atomically obtain an exclusive lock.
 
     .. note::
-        `UV_FS_O_EXLOCK` is only supported on macOS.
+        `UV_FS_O_EXLOCK` is only supported on macOS and Windows.
+
+    .. versionchanged:: 1.17.0 support is added for Windows.
 
 .. c:macro:: UV_FS_O_NOATIME
 

include/uv-win.h

@@ -664,13 +664,13 @@ RB_HEAD(uv_timer_tree_s, uv_timer_s);
 #define UV_FS_O_WRONLY       _O_WRONLY
 
 /* fs open() flags supported on other platforms (or mapped on this platform): */
-#define UV_FS_O_DIRECT       0x2000000 /* FILE_FLAG_NO_BUFFERING */
+#define UV_FS_O_DIRECT       0x02000000 /* FILE_FLAG_NO_BUFFERING */
 #define UV_FS_O_DIRECTORY    0
-#define UV_FS_O_DSYNC        0x4000000 /* FILE_FLAG_WRITE_THROUGH */
-#define UV_FS_O_EXLOCK       0
+#define UV_FS_O_DSYNC        0x04000000 /* FILE_FLAG_WRITE_THROUGH */
+#define UV_FS_O_EXLOCK       0x10000000 /* EXCLUSIVE SHARING MODE */
 #define UV_FS_O_NOATIME      0
 #define UV_FS_O_NOCTTY       0
 #define UV_FS_O_NOFOLLOW     0
 #define UV_FS_O_NONBLOCK     0
 #define UV_FS_O_SYMLINK      0
-#define UV_FS_O_SYNC         0x8000000 /* FILE_FLAG_WRITE_THROUGH */
+#define UV_FS_O_SYNC         0x08000000 /* FILE_FLAG_WRITE_THROUGH */

src/win/fs.c

@@ -439,8 +439,15 @@ void fs__open(uv_fs_t* req) {
    * does. We indiscriminately use all the sharing modes, to match
    * UNIX semantics. In particular, this ensures that the file can
    * be deleted even whilst it's open, fixing issue #1449.
+   * We still support exclusive sharing mode, since it is necessary
+   * for opening raw block devices, otherwise Windows will prevent
+   * any attempt to write past the master boot record.
    */
-  share = FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE;
+  if (flags & UV_FS_O_EXLOCK) {
+    share = 0;
+  } else {
+    share = FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE;
+  }
 
   switch (flags & (UV_FS_O_CREAT | UV_FS_O_EXCL | UV_FS_O_TRUNC)) {
   case 0:

test/test-fs.c

@@ -3061,3 +3061,60 @@ TEST_IMPL(fs_null_req) {
 
   return 0;
 }
+
+#ifdef _WIN32
+TEST_IMPL(fs_exclusive_sharing_mode) {
+  int r;
+
+  /* Setup. */
+  unlink("test_file");
+
+  ASSERT(UV_FS_O_EXLOCK > 0);
+  
+  r = uv_fs_open(NULL,
+                 &open_req1,
+                 "test_file",
+                 O_RDWR | O_CREAT | UV_FS_O_EXLOCK,
+                 S_IWUSR | S_IRUSR,
+                 NULL);
+  ASSERT(r >= 0);
+  ASSERT(open_req1.result >= 0);
+  uv_fs_req_cleanup(&open_req1);
+
+  r = uv_fs_open(NULL,
+                 &open_req2,
+                 "test_file",
+                 O_RDONLY | UV_FS_O_EXLOCK,
+                 S_IWUSR | S_IRUSR,
+                 NULL);
+  ASSERT(r < 0);
+  ASSERT(open_req2.result < 0);
+  uv_fs_req_cleanup(&open_req2);
+
+  r = uv_fs_close(NULL, &close_req, open_req1.result, NULL);
+  ASSERT(r == 0);
+  ASSERT(close_req.result == 0);
+  uv_fs_req_cleanup(&close_req);
+
+  r = uv_fs_open(NULL,
+                 &open_req2,
+                 "test_file",
+                 O_RDONLY | UV_FS_O_EXLOCK,
+                 S_IWUSR | S_IRUSR,
+                 NULL);
+  ASSERT(r >= 0);
+  ASSERT(open_req2.result >= 0);
+  uv_fs_req_cleanup(&open_req2);
+
+  r = uv_fs_close(NULL, &close_req, open_req2.result, NULL);
+  ASSERT(r == 0);
+  ASSERT(close_req.result == 0);
+  uv_fs_req_cleanup(&close_req);
+
+  /* Cleanup */
+  unlink("test_file");
+
+  MAKE_VALGRIND_HAPPY();
+  return 0;
+}
+#endif

test/test-list.h

@@ -321,6 +321,9 @@ TEST_DECLARE   (fs_write_alotof_bufs)
 TEST_DECLARE   (fs_write_alotof_bufs_with_offset)
 TEST_DECLARE   (fs_file_pos_after_op_with_offset)
 TEST_DECLARE   (fs_null_req)
+#ifdef _WIN32
+TEST_DECLARE   (fs_exclusive_sharing_mode)
+#endif
 TEST_DECLARE   (threadpool_queue_work_simple)
 TEST_DECLARE   (threadpool_queue_work_einval)
 TEST_DECLARE   (threadpool_multiple_event_loops)
@@ -832,6 +835,9 @@ TASK_LIST_START
   TEST_ENTRY  (fs_read_write_null_arguments)
   TEST_ENTRY  (fs_file_pos_after_op_with_offset)
   TEST_ENTRY  (fs_null_req)
+#ifdef _WIN32
+  TEST_ENTRY  (fs_exclusive_sharing_mode)
+#endif
   TEST_ENTRY  (get_osfhandle_valid_handle)
   TEST_ENTRY  (threadpool_queue_work_simple)
   TEST_ENTRY  (threadpool_queue_work_einval)